This invention relates generally to a method and apparatus for concentrating aqueous solutions and more particularly concerns an apparatus and method for concentrating aqueous solutions of hygroscopic organic liquids having boiling points higher than the normal boiling point of water. Typical of the organic substances suitable for concentrating according to the present invention are the polyhydroxy organic compounds such as propylene glycol, ethylene glycol, diethylene glycol and triethylene glycol.
The present invention has particular usefulness in conjunction with recirculating aqueous solutions of these hygroscopic organic liquids which become progressively more dilute in dehumidifying and certain anti-freeze applications. Typical of the latter are systems where the solutions are used to flood air cooling coils maintained at subfreezing temperatures to prevent the coils from icing up due to moisture in the air, while absorbing some of this moisture into the solution as a diluent. In this application, as dilution progresses, the anti-freeze effectiveness of the solution decreases to a point where removal of water and reconcentration of the solution is required for further efficient operation of the air cooling operation.
While various techniques have been suggested and are in use for concentrating aqueous anti-freeze solutions, these techniques have many serious shortcomings. For example, since most of these techniques were developed in a bygone era of cheap resources, they are not energy efficient and may require excessive amounts of externally supplied cooling water and steam or electricity. Furthermore, most of these prior concentrating techniques permit the loss of excessive amounts of the hygroscopic organic liquid.
A typical prior approach to concentrating aqueous solutions of hygroscopic organic liquid involves heating these solutions at nearly atmospheric pressure in a simple distillation process to boil off excess water thereby returning the spent solution to the desired concentration of the hygroscopic organic liquid. This approach requires significant amounts of energy to raise the temperature of the hygroscopic organic liquid to boiling. As distillation proceeds, significant amounts of the hygroscopic organic liquid are lost along with the water vapor. Furthermore, additional energy is required to cool the concentrated solution to a temperature low enough to permit it to be used in the desired application. Finally, such high temperature processes tend to degrade many hygroscopic organic liquids.
Another prior art concentrating method entails passing a current of a carrier gas such as air over an extended surface of the solution to be concentrated while maintaining this solution at a relatively low temperature. According to this method, which avoids the high temperatures of distillation, significant amounts of energy are nevertheless required to heat the hygroscopic organic liquid in order to evaporate the higher vapor pressure water into the carrier gas. Furthermore, this technique still loses significant amounts of the hygroscopic organic liquid. Apparatus for carrying out this method usually involves the use of packings and elaborate baffle systems, which can be expensive, bulky, and difficult to build and maintain.
Yet another prior art concentrating apparatus utilizes a series of spray nozzles in lieu of the packing or baffle systems described above to maximize the area of contact between the solution being treated and the carrier gas which is to carry away the unwanted water vapor. U.S. Pat. No. 2,778,782, which illustrates one such device, also utilizes these spray nozzles to draw a slow moving stream of fresh air through the apparatus to act as the carrier gas. This air stream would be expected to produce an insignificant pressure drop of about 0.01 inches of water which is not significantly different from atmospheric pressure and will not noticeably contribute to vaporizatioh at the spray nozzles. Even in systems utilizing fans to push or draw outside air through the system as a carrier gas, pressure drops of only about 1.5 inches of water or about 0.1 of a normal barometric pressure storm change would be produced. Again, this pressure change is not significantly different from atmospheric.